I've made something similar, albeit with less I/O ports (I needed to had a bunch of buttons to an ESP8266). I used an Atmega 328 linked by I2C to the ESP. Although I programmed the Atmega myself, this library should be useful to you : github.com/PetrOsipov/A...

I identify as a male praying mantis and I can confirm it sucks !

By compatible, I mean the Belkin firmware can be replaced in some case by Openwrt. But indeed I don't expect people to be able to modify their devices, apart from a few adventurous tinkerers. I've modified a few not-so-Smartplugs to run better software but I'm a nerd :)

I read that some Belkin devices are compatible with Openwrt. For the not-compatible ones, you could perhaps gut the electronic board and replace it with an ESP flashed with Tasmota.

Or the opposite ... My own pet peeve is the lack of documentation and comprehensive examples in the libraries.

If you mean an sd card reader, you could use a micro-sd card adapter with a few resistors, see here : eecs.blog/diy-arduino-...

If you add an USB wifi dongle, you can install openwrt on it and use it as a wifi router

I bought a nesting box that basically has a baby-monitoring camera above the nest (in my case, a Foscam C2). Those cameras are round and flat, ideal to install in a small enclosure. As long as your box is watertight, there shouldn't be any problem (I have mine for 3-4 years already)

Je suis passé récemment à une machine à état pour mes programmes et cela aide à clarifier le code, en effet. Séparer les fonctions dans différents fichiers aide aussi (genre un fichier pour l'interface web, un autre pour l'affichage, etc ...)

A long time ago, someone posted a how-to for illuminating a USS Enterprise model kit with a bunch of LEDs and an Arduino, here : www.britmodeller.com/forums/index...
It could inspire you.